1. Field of the Invention
The present invention relates to laser mirror structures. More specifically, it relates to mirror structures suitable for use in vertical cavity surface emitting lasers.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides for current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by forming an oxide layer. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. In particular, in some applications the available prior art distributed Bragg Reflectors (DBR) are significantly less than optimal. To understand why this is so it is beneficial to consider DBRs in more detail.
In many applications a DBR must be both highly reflective and highly electrically conductive. In fact, when used in a VCSEL a DBR must be particularly reflective so as to reduce optical losses to such a degree that laser operation is enabled. Reflectivity is achieved by stacking material layers having significantly different indexes of refraction, for example, by stacking alternating layers of AlAs and GaAs. Such stacked layers can produce an optical standing wave within the DBR.
While the optical performance of stacked AlAs and GaAs is very good, an abrupt junction between an AlAs layer and a GaAs layer would form a high barrier to current flow. To reduce that barrier, the layers of AlAs and GaAs are joined using a transition region in which the material composition gradually changes from AlAs to GaAs. Furthermore, in most VCSELs the DBR layers are doped to provide free carriers that reduce electrical resistance. The result is a structure that, ideally, has high reflectivity combined with both low optical absorption and low electrical resistance.
In practice, optical absorption increases with increasing electric field strength, increasing wavelength, and increasing doping levels. Furthermore, p-type dopants tend to have higher optical absorption than n-type dopants. On the other hand, electrical resistance is relatively unaffected by electrical field strength and optical wavelength, yet decreases with increasing doping levels. But, p-type carriers (holes) have much lower mobilities than n-type carriers (electrons). Therefore, as the wavelength increases, such as with VCSELs that output light at 1300, 1310, or 1550 nm, obtaining both low optical absorption and low electrical resistance is difficult. This is because long optical wavelengths tend to be highly absorbed by the free carriers that reduce the electrical resistance. Such is particularly true in top DBRs, which are usually p-doped. The lower mobility of the p-carriers and the higher optical absorption of p-dopants tend to reduce the performance of such top DBRs. Therefore, a conflict occurs in prior art long wavelength VCSELs: to reduce electrical resistance the free carrier concentration should be high, but to reduce light absorption the free carrier concentration should be low.
In addition, it should be understood that DBRs produce optical standing waves that are characterized by spatially dependent electrical fields. That is, the electric field strength varies across the DBR's thickness. Additionally, the materials that form a DBR strongly impact on the thermal characteristics of a VCSEL. Binary phase materials, such as AlAs and GaAs tend to have very good thermal conductivity. Thus, heat flows across AlAs and GaAs stacks very well. However, the transition region, which is characterized by three materials, has a significantly lower thermal conductivity. This is because the crystalline structure of the transition region is highly distorted, which reduces thermal conductivity.
Because of the foregoing, DBRs used in prior art VCSELs have problems with excessive optical absorption, relatively poor thermal conductivity, and relatively high electrical resistance, particularly when producing long wavelength light. Therefore, a new distributed Bragg reflector that has relatively low light absorption and relatively low electrical resistance, particularly at long wavelengths and in top DBRs, would be beneficial. Even more beneficial would be a new distributed Bragg reflector that has relatively low light absorption, relatively low electrical resistance, and relatively good thermal conductivity, particularly at long wavelengths and in top DBRs.